The invention is an improvement over U.S. Pat. No. 4,389,825, which issued June 28, 1983. That invention employed a stress-relieving slot (SRS) to increase the pressure-resisting capacity of a prestressed concrete pressure vessel (PCPV).
The application of external prestressing causes tangential stress in a pressure-vessel wall to peak at the inner surface. This peak stress controls the design of the vessel and limits its capacity. If the peak stress at the 23 inner surface is moved toward the center of the wall, where the allowable concrete stress can be enhanced due to the confined state of the concrete, the capacity of the same vessel is also increased. The purpose of the invention of U.S. Pat. No. 4,389,825, the stress-relieving slot method, was to achieve this by accomplishing the following:
1. Lower the peak tangential compressive stress (TCS) the inner surface due to circumferential prestressing P.sub.0 on the exterior surface, and
2. Move this peak stress to the inside of the wall, concrete strength is enhanced due to the confinement of concrete.
The ASME Code allows an increase of up to 2.7 times the normal concrete strength when the concrete is confined. Thus, theoretically, if concrete of 5,000 psi strength is used in the construction of a thick-walled PCPV, it can be stressed up to 13,500 psi at or near the wall center where the concrete confined.
The prestressed concrete pressure vessel or PCPV is stressed externally or in the external region of the cylindrical wall in order to produce tangential compressive stress or TCS in the wall, with which to counter, and neutralize if possible, the tangential tensile stress (TTS) which takes place when internal pressure P.sub.i is introduced in the cavity within the vessel. The design aim of the U.S. Pat. No. 4,389,825 was therefore to match the relocated peak TCS the wall due to P.sub.0, with the TTS due to P.sub.i. The distribution of both stresses assumes more or less the same form across the wall. Any difference between the two should not be so large that it would require a large amount of tangential reinforcement in a wall. Excessive reinforcement and steel congestion is undesirable, even impractical, depending on the amount of the excess.
However, the effectiveness of the method of U.S. Pat. No. 4,389,825 was limited because the depth into the wall which peak stress could be moved was a function of the depth of the slots. The slots themselves were limited in depth, because they reduced the effective structural depth of the wall.
An object of the present invention is to close the gap between the TCS and TTS stresses.
Whereas the earlier method utilized only vertical radial slots on the inner surface of the cylindrical pressure vessel to force the peak tangential stress to move toward the center of the wall, the present invention adds to that structure a new layered construction.